Progress in the semiconductor industry has been following the trend of Moore's Law that was proposed in 1965 by then Intel's cofounder Gordon Moore. This trend requires that the capability of integrated circuits (IC) or, in general, semiconductor chips double every 18 months.
Along with such advances comes various design challenges. One of the often overlooked challenges is that of heat dissipation. Most often, this phase of design is neglected or added as a last minute design before the components are produced. According to the second law of thermodynamics, the more work that is performed in a closed system, the higher entropy it will attain. With the increasing power of a central processing unit (CPU), the larger flow of electrons produces a greater amount of heat. Therefore, in order to prevent the circuitry from shorting or burning out, the heat resulting from the increase in entropy must be removed. Some state-of-the-art CPU's have a power of about 70 watts (W) or more. For example, a CPU made with 0.13 micrometer technology may exceed 100 watts. Current methods of heat dissipation, such as by using metal (e.g., Al or Cu) fin radiators, and water evaporation heat pipes, will be inadequate to sufficiently cool future generations of CPUs.
Recently, ceramic heat spreaders (e.g., AIN) and metal matrix composite heat spreaders (e.g., SiC/Al) have been used to cope with the increasing amounts of heat generation. However, such materials have a thermal conductivity that is no greater than that of Cu, hence, their ability to dissipate heat from semiconductor chips is limited.
A typical semiconductor chip contains closely packed metal conductors (e.g., Al, Cu) and ceramic insulators (e.g., oxide, nitride). The thermal expansion of metal is typically 5-10 times that of ceramics. When the chip is heated to above 60° C., the mismatch of thermal expansions between metal and ceramics can create microcracks. The repeated cycling of temperature tends to aggravate the damage to the chip. As a result, the performance of the semiconductor will deteriorate. Moreover, when temperatures reach more than 90° C., the semiconductor portion of the chip may become a conductor so the function of the chip is lost. In addition, the circuitry may be damaged and the semiconductor is no longer usable (i.e. becomes “burned out”). Thus, in order to maintain the performance of the semiconductor, its temperature must be kept below a threshold level (e.g., 90° C.).
A conventional method of heat dissipation is to contact the semiconductor with a metal heat sink. A typical heat sink is made of aluminum that contains radiating fins. These fins are attached to a fan. Heat from the chip will flow to the aluminum base and will be transmitted to the radiating fins and carried away by the circulated air via convection. Heat sinks are therefore often designed to have a high heat capacity to act as a reservoir to remove heat from the heat source.
Alternatively, a heat pipe may be connected between the heat sink and a radiator that is located in a separated location. The heat pipe contains water vapor that is sealed in a vacuum tube. The moisture will be vaporized at the heat sink and condensed at the radiator. The condensed water will flow back to the heat sink by the wick action of a porous medium (e.g., copper powder). Hence, the heat of a semiconductor chip is carried away by evaporating water and removed at the radiator by condensing water.
Although heat pipes and heat plates may remove heat very efficiently, the complex vacuum chambers and sophisticated capillary systems prevent designs small enough to dissipate heat directly from a semiconductor component. As a result, these methods are generally limited to transferring heat from a larger heat source, e.g., a heat sink. Thus, removing heat via conduction from an electronic component is a continuing area of research in the industry.
One promising alternative that has been explored for use in heat spreaders is diamond-containing materials. Diamond can carry away heat much faster than any other material. The thermal conductivity of diamond at room temperature (about 2000 W/mK) is five times higher than copper (about 400 W/mK) and eight times that of aluminum (250 W/mK), the two fastest metal heat conductors commonly used. Moreover, the thermal diffusivity of diamond (12.7 cm2/sec) is eleven times that of copper (1.17 cm2/sec) or aluminum (0.971 cm2/sec). The ability for diamond to carry away heat without storing it makes diamond an ideal heat spreader. In contrast to heat sinks, a heat spreader acts to quickly conduct heat away from the heat source without storing it. Table I shows various thermal properties of several materials as compared to diamond (values provided at 300 K).
TABLE 1ThermalThermalConductivityHeat CapacityExpansionMaterial(W/mK)(J/cm3 K)(ppm/K)Copper4013.4416.4Aluminum2372.4424.5Molybdenum1382.5747.5Gold3172.4914.5Silver4292.4718.7Tungsten Carbide952.955.7Silicon1481.662.6Diamond (IIa)2,3001.781.4
In addition, the thermal expansion coefficient of diamond is one of the lowest of all materials. The low thermal expansion of diamond makes joining it with low thermally expanding silicon semiconductor much easier. Hence, the stress at the joining interface can be minimized.
In recent years diamond heat spreaders have been used to dissipate heat from high power laser diodes, such as that used by laser diodes to boost the light energy in optical fibers. However, large area diamonds are very expensive; hence, diamond has not been commercially used to spread the heat generated by CPUs. In order for diamond to be used as a heat spreader, its surface must be polished so it can make an intimate contact with the semiconductor chip. Moreover, its surface may be metallized (e.g., by Ti/Pt/Au) to allow attachment to a conventional metal heat sink by brazing.
Many current diamond heat spreaders are made of diamond films formed by chemical vapor deposition (CVD). One example of raw CVD diamond films are now sold at over $10/cm2, and this price may be doubled when it is polished and metallized. This high price would prohibit diamond heat spreaders from being widely used except in those applications (e.g., high power laser diodes) where only a small area is required or no effective alternative heat spreaders are available. In addition to being expensive, CVD diamond films can only be grown at very slow rates (e.g., a few micrometers per hour); hence, these films seldom exceed a thickness of 1 mm (typically 0.3-0.5 mm). However, if the heating area of the chip is large (e.g., a CPU), it is preferable to have a thicker (e.g., 3 mm) heat spreader.
In addition to diamond products produced using CVD methods, attempts have been made to form heat spreaders using a mass of particulate diamond or “polycrystalline diamond” (PCD). Specific examples of such devices are found in U.S. Pat. No. 6,390,181, and U.S. Patent Application Publication No. 2002/0023733, each of which is incorporated herein by reference. Typically, a PCD product (or “compact”) is formed by sintering diamond particles under high-pressure, high-temperature (HPHT) conditions to using cobalt as a sintering aid. Alternatively, silicon or its alloy can be used to cement diamond particles together, as described in U.S. Pat. Nos. 4,124,401 and 4,534,773. Diamond particles used in typical sintering processes have a particle size in the micron range. Thus, PCD compacts typically have extensive grain boundaries with a low conductivity second phase surrounding individual grains. Such PCD compacts are of limited use in the field of heat spreaders because of their limited physical capacity to transfer or conduct heat.
As such, cost effective systems and devices that are capable of effectively conducting heat away from a heat source, continue to be sought through ongoing research and development efforts.